Polarization independent optical isolators, attenuators, circulators and switches often use birefringent plates (also referred to as “walk-off” crystals) to divide an optical beam into parallel polarized beams and to combine parallel polarized beams into a single beam.
FIG. 1A shows a birefringent plate of yttrium orthovanadate in which an unpolarized beam of wavelength 1550 nm is so divided. The optic axis of the birefringent plate is in the plane of the drawing and oblique to the input face at an angle of 45 degrees giving an angular beam separation of 5.7 degrees. If separated beams pass through the plate in an opposite direction they may be combined into a single beam.
U.S. Pat. No. 5,864,428 discloses means by which a beam may be divided into parallel polarized beams by use of birefringent prisms. In FIG. 1B a beam of wavelength 1550 nm passes through a 20 degree wedge of yttrium orthovanadate with its optic axis arranged normal to the plane of the drawing so that the beam is separated into polarized beams with an angular beam separation of 5.6 degrees. Component beams then pass through a similar prism to form parallel beams.
As birefringent material is expensive, use of prisms may be advantageous in that the amount of birefringent material may be reduced, so reducing cost. Also, with a space between prisms, one or more polarization rotators may be arranged between prisms to increase optical isolation while maintaining an output free from polarization mode dispersion.
It is therefore an object of this invention to provide a device such as an optical isolator, attenuator, circulator or switch which may be conservative in the use of birefringent material.
It is also an object of the invention to provide a device such as an optical isolator, attenuator, circulator or switch which may be free from polarization mode dispersion and conservative in the use of birefringent material.
It is a further object of the invention to provide a device such as an optical isolator, attenuator, circulator or switch with high optical isolation which may be free from polarization mode dispersion and conservative in the use of birefringent material.
It is a further object of the invention to provide an improved optical isolator.
It is a further object of the invention to provide an improved optical attenuator.
It is a further object of the invention to provide an improved optical circulator.
It is a further object of the invention to provide an improved optical switch.
To this end in accordance with the invention the device may be characterised as an optical isolator, attenuator, circulator or switch comprising a first combination of birefringent prisms with parallel optic axes for dividing an optical input beam into polarized beams, a second combination of birefringent prisms with parallel optic axes for combining polarized beams into an output beam, a first polarization changer disposed between said first combination of birefringent prisms with parallel optic axes for dividing an optical input beam into polarized beams and said second combination of birefringent prisms with parallel optic axes for combining polarized beams into an output beam, a second polarization changer disposed between at least first and last prisms of said first combination of birefringent prisms with parallel optic axes for dividing an optical input beam into polarized beams or first and last prisms of said second combination of birefringent prisms with parallel optic axes for combining polarized beams into an output beam, wherein each birefringent prism of each said combination of birefringent prisms has non parallel input and output faces.